A belt formed by juxtaposing a plurality of metal pieces called an “element” or a “block” in a circular manner, and by fastening the juxtaposing metal pieces by a metal belt member called a “ring” or a “hoop” is known in the prior art to be used in a continuously variable transmission. The belt of this kind is adapted to transmit the torque by a pushing force among the metal pieces thus juxtaposed to be contacted to one another. Specifically, the metal pieces existing in the groove of a drive pulley are sequentially pushed out of the groove by a rotation of the pulley while pushing the metal pieces in front of those metal pieces. The metal pieces thus being pushed forward are eventually entered into a groove of a driven pulley. As a result, the torque of the drive pulley is transmitted to the driven pulley by such advancement of the metal pieces.
An example of the belt thus structured is disclosed in Japanese Patent Laid-Open No. 2002-39280. The driving belt taught by Japanese Patent Laid-Open No. 2002-39280 comprises a transverse element, and both side faces thereof are to be contacted with pulleys in case the driving belt is disposed between the pulleys. The transverse elements are fastened by two endless rings functioning as a band package. Therefore, the transverse element consists of: a first part, which extends under band package, and which is clamped by the pulleys; a second part, which is erected in the width center of the first part to protrude upwardly; and a third part, which extends transversely from the upper end of the second part. Namely, the first part serves as a main body comprising a saddle face on which the ring is disposed, the second part serves as a neck portion protruding upwardly from the saddle face, and the third part serves as a stopper portion covering the saddle face partially from above. In addition, the transverse element taught by Japanese Patent Laid-Open No. 2002-39280 further comprises recessed portions formed by rounding both of the side faces of the neck portions toward the width center. According to the teachings of Japanese Patent Laid-Open No. 2002-39280, such recessed portion is formed to prevent interference between the lateral end of the ring and the neck portion, and stress concentration on those portions. For this purpose, a concave curvature of the inside surface of the recessed portion is rounded to be constant.
The recessed portion of the belt taught by Japanese Patent Laid-Open No. 2002-39280 is thus formed to prevent the stress concentration. For this purpose, the radius of the concave curvature of the inside surface has to be enlarged according to the envisaged stress. That is, in order to relaxing such stress concentration certainly, the curvature radius of the recess has to be enlarged. However, according to the teachings of Japanese Patent Laid-Open No. 2002-39280, the recessed portion is formed on both sides of the neck portion by rounding from an innermost portion of the saddle face to an innermost portion of a lower face of the stopper portion through the side face of the neck portion. Therefore, if the curvature radius of the recess is enlarged, a width of the neck portion is thereby narrowed.
In order to avoid the above-explained disadvantage, it may be possible to enlarge the curvature radius of the recess while keeping sufficient width of the neck portion. In this case, however, a total width of the element has to be widened to ensure sufficient widths of the saddle faces and the stopper portion. As a result, the belt has to be enlarged entirely. Thus, according to the structure taught by Japanese Patent Laid-Open No. 2002-39280, it would be difficult to relax the stress concentration if the curvature radius of the recess is reduced for the purpose of avoiding enlargement of the element. In other words, it is inevitable for the element taught by Japanese Patent Laid-Open No. 2002-39280 to be enlarged if the durability of the element is enhanced while relaxing the stress concentration.
As described, according to the teachings of Japanese Patent Laid-Open No. 2002-39280, the third part serves as a stopper portion for preventing disengagement of the rings from the element by holding the rings from above. For example, the upper face of the ring is contacted strongly with the stopper portion in case the element is getting out of the groove of the pulley. In this case, bending load may be applied to the second part functioning as a neck portion. However, Japanese Patent Laid-Open No. 2002-39280 fails to disclose a countermeasure against such bending load to improve strength or durability of the element.